This invention relates to a method of crystallizing materials from aqueous crystallization media employing microwave radiation. In one embodiment, the crystallization method herein is utilized in the manufacture of porous crystalline materials which are especially useful for the catalysis of a wide variety of chemical conversion processes.
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline metallosilicates having a definite crystalline structure as determined by X-ray diffraction within which there are a number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of large dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline metallosilicates. For example, in the case of the aluminosilicates, these materials can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra which are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate or other metallosilicate zeolite by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic porous, crystalline metallosilicate zeolites. The zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite L (U.S. Pat. No. 3,130,006), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite beta, (U.S. Pat. No. 3,308,069), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-4 (Great Britain Pat. No. 1,117,568), zeolite ZSM-5 (U.S. Pat. No. 3,702,886, now U.S. Pat. No. Re. 29,948), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-22, zeolite, ZSM-23 (U.S. Pat. No. 4,076,842), zeolite ZSM-34, zeolite ZSM-35 (U.S. Pat. No. 4,016,245), zeolite ZSM-39 (U.S. Pat. No. 4,259,306), zeolites ZSM-21 and ZSM-38 (U.S. Pat. No. 4,046,859), ZSM-48 (U.S. Pat. No. 4,375,573), ZSM-51 (U.S. Pat. No. 4,568,654), zeolite EU-1 (European Patent Application 0042 226), zeolite EU-2 (UK Patent Application No. GB 2077709 A), zeolite EU-4 (European Patent Application No. 0 063 436), and zeolites NU-6(1) and NU-6(2) (U.S. Pat. No. 4,397,825), merely to name a few. Zeolites containing a framework element other than, or in addition to, aluminum, e.g., boron, iron, titanium, zirconium, germanium, gallium, etc., are known from, inter alia, U.S. Pat. Nos. 3,328,119; 3,329,480; 3,329,481; 4,414,423 and 4,417,088.
A summary of the channel description and composition of these and other zeolite catalysts is set forth below.
______________________________________ ZEOLITE CHANNEL AND COMPOSITION SUMMARY Silica/Alumina Zeolite Isotypes.sup.a Ratio.sup.b Channel Description.sup.c ______________________________________ Medium Pore ZSM-35 Ferrierite &gt;8 2d, 4.3 .times. 5.5-3.4 .times. 4.8 A ZSM-22 &gt;20 1d, 4.5 .times. 5.5 A ZSM-23 40-250 1d, 4.5 .times. 5.6 A ZSM-48 &gt;25 1d, 5.3 .times. 5.6 A ZSM-5 &gt;12 3d, 5.4 .times. 5.6-5.1 .times. 5.5 A ZSM-11 20-90 3d, 5.4 .times. 5.6 A ZSM-50 20-100 1d, 4.5 .times. 6.3 A ZSM-12 20-100 1d, 5.7 .times. 6.1 A Heulandite 7 2d, 4.4 .times. 7.2-4.0 .times. 5.5 A Offretite 7 3d, 6.4-3.6 .times. 5.2 A Large Pore Mordenite 10 1d, 5.8 .times. 7.0 A Beta 5-100 3d, 6.2 .times. 7.6-5.5 .times. 6.3 A Gmelinite 4 3d, 7.0- 3.6 .times. 3.9 A Linde Type L 6 1d, 7.1 A ZSM-4 Omega 3-20 1d, 7.4 A Mazzite Faujasite X,Y 2-6 3d, 7.4 A ______________________________________ .sup.a Common isotypes. .sup.b Typical composition ranges. .sup.c Dimensionality followed by simplified listing of limiting pore sizes.
Synthetic zeolites are generally prepared by providing an aqueous solution of the desired oxides and other required components of the crystallization reaction medium and thereafter crystallizing the zeolite under heat and pressure. For specific synthesis and post-crystallization processing conditions for a particular zeolite, reference may be made to the extensive literature on the subject and, in particular, the U.S. patents referred to above, the contents of which are incorporated by reference herein.
Other porous crystalline materials which are not zeolitic but which also exhibit catalytic adsorption and/or catalytic properties characteristic of the zeolites are known. U.S. Pat. Nos. 4,310,440 and 4,385,994 both describe porous crystalline aluminophosphates and U.S. Pat. No. 4,440,871 describes porous crystalline silicoaluminophosphates which are useful as adsorbents and as catalysts for a variety of hydrocarbon conversions. U.S. Pat. No. 4,567,029 describes porous crystalline metal aluminophosphates containing as lattice constituents in addition to AlO.sub.2 and PO.sub.2 structural units, one or a mixture of two or more of the metals Mg, Mn, Co and Zn in tetrahedral coordination with oxygen atoms. The contents of these patents are incorporated by reference herein.
The aluminophosphates are prepared by hydrothermal crystallization of a reaction mixture prepared by combining a reactive source of phosphate, alumina and water and at least one structure-directing or templating agent which can include an organic amine and a quarternary ammonium salt. The silicoaluminophosphates are synthesized by hydrothermal crystallization from a reaction mixture containing reactive sources of silica, alumina and phosphate, an organic templating, i.e., structure-directing agent, preferably a compound of an element of Group VA of the Periodic Table, and optionally an alkali metal. In the case of both types of material, the reaction mixture is placed in a reaction vessel inert toward the reaction system and heated until crystallized, usually a period of from about 2 hours to about 2 weeks. The solid crystalline reaction product is then recovered by any convenient method such as filtration or centrifugation.
While it is known from Roussy et al., "Selective Energy Supply To Adsorbed Water and Nonclassical Thermal Process during Microwave Dehydration of Zeolite," J. Phys. Chem. 85. 2199-2203 (1981) that microwave energy can be used to desorb water from a zeolite, the use of microwave energy in the crystallization of synthetic zeolites is believed to be novel.